In a typical application a micro-circuit mixer is connected via waveguide to an aerial collecting dish or antenna. The micro-circuit usually consists of a dielectric support plate having patterned conductors and bonded semiconductor mixer components -- e.g., diodes -- on one surface and may also be metallised over all or part of the other surface. The positioning of the above components is extremely critical. The micro-circuit is usually mounted in the waveguide cavity, or else is connected to the waveguide by a specially designed transition. The positioning of the circuit is also critical. Such assemblies are difficult to set up, fragile and expensive. Positioning of the components and of the circuit is difficult and costly.